Within our highly technological and industrial world, the oxides of nitrogen have become recognized as major pollutants of the air and the environment. Perhaps the best known of these pollutants are the NO.sub.x compounds such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2) which originate in the exhaust gases produced by the combustion of fossil fuels in automobile engines, jet engines used in propulsion of aircraft, gas-turbine power generators, and steam power-plants and space-heating systems based on the burning of natural gas, petroleum products, and other fossil fuels. Nitric oxide is a known precursor of ozone; and in combination with ozone and hydrocarbon vapors leads to photochemical smog, a health and ecological hazard. Nitrogen dioxide is a precursor of acid rain [Calvert, S. and H.M. England, Handbook Of Air Pollution Technology, John Wiley & Sons, Inc., N.Y., 1984, pp 71-81].
The decomposition of both NO and NO.sub.2 into molecular oxygen and molecular nitrogen is thermodynamically spontaneous at 25.degree. C. [.DELTA.G.degree.=-20.72 and -12.39kcal/mole for NO and NO.sub.2 respectively]but uncatalyzed gas-phase decomposition does not proceed at a significant rate at this temperature. Rather, the position of equilibrium among NO, NO.sub.2, N.sub.2, and O.sub.2 shifts towards NO with increasing temperature above 1,000.degree. C. as the rate of equilibration in the gas-phase becomes ever faster. Thus, NO is a reaction by-product of many high-temperature reactions generally; and is a specific by-product of the high-temperature combustion of natural gas and other organic fuels even when such fuels do not contain chemically bound nitrogen Subsequently, the partial oxidation of NO into NO.sub.2 occurs typically when NO-containing effluent gas from combustion is mixed with air thereby producing a mixture of oxides of nitrogen commonly designated as NO.sub.x wherein x is a positive number
It is noteworthy that in spite of extensive research, no practically effective catalyst or catalytic reaction at ordinary or elevated temperatures for the decomposition of NO into molecular nitrogen and oxygen has been reported [Harrison et al., "Catalysis Of Reactions Involving The Reduction Or Decomposition Of Nitrogen Oxides", in Catalysis, Volume 5, (G.C. Bond and G.W. webb, editors) The Royal Society Of Chemistry, London, 1982, pages 127-171; Mobley, J.D. and K.J. Lim, "Control Of Gases By Chemical Reaction," and E. DeKiep and D.J. Patterson, "Emission Control In Internal Combustion Engines," in Handbook Of Air pollution Technology, John Wiley and Sons, lnc., N.Y., 1984, pages 203-213 and 489-512, and the references cited therein]. Instead, present technology controls NO.sub.x and oxides of nitrogen in general, not by decomposition, but rather by reducing these pollutants to molecular nitrogen. This is accomplished in several ways: control of emissions from stationary sources accomplishes NO.sub.x reduction using NH.sub.x as in the non-catalytic "DeNO.sub.x " process developed by Exxon Corporation for treatment of stack gases [U.S. Pat. No. 3,900,554]; or in analogous catalytic processes using NH.sub.3 and a catalyst such as V.sub.2 O.sub.5 [Mobley and Lim, op. cit., pp 206-209]. In contract, the removal of NO.sub.x from the exhaust gases stemming from internal combustion engines is accomplished by reducing the exhaust to N.sub.2 using CO and/or unburned hydrocarbon catalyzed by rhodium DeKiep and patterson, supra]. In addition, a recent report describes the use of cyanuric acid,(HOCN).sub.3, to reduce NO into N.sub.2 with concomitant production of carbon monoxide as a practical means of treating diesel exhaust [Perry, R.A. and D.L. Siebers, Nature 324:657-658 (1986)].
A variety of problems, however, exists: while large scale industrial processes use NH.sub.3 to reduce oxides of nitrogen, one recognizes that the use of NH.sub.3 as a reactant involves the major risk of substituting one hazardous pollutant (NH.sub.3) for another (NO.sub.x). Moreover, both rhodium and platinum used currently in automotive antipollution units are not only very expensive but must be obtained from the Soviet Union or from South Africa, sources which cannot be regarded today as entirely reliable Similarly, the use of cyanuric acid merely substitutes CO in place of NO in the effluent gas and requires periodic replenishment of that reagent.
In comparison, a large part of the present knowledge of catalysts useful in decomposition originates from the work of Winter on thermal catalytic decomposition of NO using metal oxides [E.R.S. Winter, J. Catal. 22:158-170 (1971); E.R.S. Winter, J. Catal. 34:44-444 (1974)]. The reported results describe a variety of metal oxides which are promising because they display thermal catalytic activity at temperatures ranging from 33.degree.-870.degree. C. The activity of these metal oxides, however, was solely as thermal catalysts using a thermal decomposition reaction.
Overall, therefore, there is a clear and generally recognized need for processes and a technology which is able to decompose NO.sub.x compounds and oxides of nitrogen generally into environmentally compatible products using catalytic methods which do not suffer from the drawbacks and hazards associated with conventionally known techniques and industrial processes. The development of a photopromoted, solid-catalyzed method for decomposition of nitric oxide and other oxides of nitrogen would therefore be recognized as a major improvement and long sought for advance in controlling pollutants originating by the combustion of fossil fuels.